This proposal deals with two aspects of reflex motor control--the muscle spindle receptor, one of the principal sources of afferent information for such control, and the central processing of sensory data. There are two corresponding basic objectives: a) To extend previously acquired quantitative descriptions of the mammalian muscle spindle in attempt to explain their behavior in terms of basic physical mechanisms, and b) to model a specific neuronal circuit, Clarke's nucleus, to discover the functional role of this second order sensory pathway to the cerebellum. The modeling employs two complimentary processes, analysis and simulation, to systematically describe the connectivity and functional role of a given neuronal circuit. It is anticipated that the technique will have wide potential application in experimental neurophysiology.